Hydrophobic synthetic resin sheets such as styrenic resin sheets are inferior in anti-fogging property and antistatic property. Therefore, when such a sheet is shaped or molded into a tray to store a food, water vapor is attached to the surface of the tray as small droplets due to temperature and humidity changes. As a result, the tray gets clouded (misty or dim) thereby deteriorating transparency.
Japanese Patent Application Laid-Open No. 115781/1978 (JP-53-115781A) proposes the attachment of an anti-fogging agent (such as a surfactant or a polyvinyl alcohol) and a silicone oil to a thermoplastic resin film in which the surface of the film is treated by a corona discharge to give the surface tension of 40 to 55 dyn/cm. Japanese Patent Publication No. 62538/1988 (JP-63-62538B) discloses a method for coating a styrenic resin film with an aqueous solution comprising a sucrose fatty acid ester, a non-modified polyvinyl alcohol having a polymerization degree of not more than 800, and a silicone emulsion at a specific proportion. Japanese Patent Application Laid-Open No. 309785/1998 (JP-10-309785A) discloses a styrenic resin sheet in which one side of the sheet is coated with a mixture of a sucrose fatty acid ester and a methyl cellulose, the coated layer is further coated with a silicone oil, and the other side of the sheet is coated with a silicone oil. Furthermore, Japanese Patent No. 3241797 description discloses that at least one side of a polymer film is coated with a surface-treating agent comprising a sucrose fatty acid ester, a silicone emulsion, and a polysaccharide and/or a hydrophilic polymer (excluding a polyvinyl alcohol).
These anti-fogging sheets or films containing a silicone oil are excellent in antiblocking property. However, anti-fogging property of these anti-fogging sheets is not enough. That is, the anti-fogging sheets are, for example, low in anti-fogging property (anti-fogging property at high temperature) against water vapor generated from a high-temperature content which is put in a tray using the sheets, in particular low in anti-fogging property (anti-fogging property at low temperature) against water vapor or dew condensation when a content containing water (e.g., a food) is charged in a tray using the sheets and kept at a low temperature. Further, anti-fogging property is significantly deteriorated by rolling up the sheet, or shaping the sheet into a tray. That is, a resin sheet for shaping into a tray (e.g., a styrenic resin sheet) is, for example, formed by melt-kneading a resin, extrusion-molding the kneaded product to a sheet form, biaxially drawing the resultant sheet, coating an anti-fogging agent on the sheet, and drying the coated sheet to roll up in a roll form. Moreover, when shaping or molding a tray, the tray is shaped or molded by paying out the resin sheet from the roll, and thermoforming the sheet. However, along with rolling up of the resin sheet, the anti-fogging agent of the sheet is contacted with the non-treated surface of the resin sheet and transferred thereto. Further, in the process for shaping or molding the tray, probably because the anti-fogging agent is transferred by contacting with a heated member such as a hot plate, anti-fogging property or antiblocking property of the resin sheet or tray is remarkably decreased. In particular, in the case shaping or molding a tray by deep drawing, anti-fogging property is significantly deteriorated. Moreover, a molding machine to be used is stained with the anti-fogging agent. Further, a relatively large amount of silicone oil is necessary for increasing mold-releasing property of the anti-fogging sheet.
Japanese Patent Application Laid-Open No. 171052/2001 (JP-2001-171052A) discloses a styrenic resin sheet obtained by coating at least one side of the sheet with an antistatic agent at a coating amount of 2 to 30 mg/m2, wherein the antistatic agent contains a fatty acid amide and a polyoxyethylene-polyoxypropylene block copolymer at a specific ratio. This document also describes that the antistatic agent further contains a silicone oil, and the coating amount of the silicone oil is 1 to 25 g/m2. Japanese Patent Application Laid-Open No. 12686/2002 (JP-2002-12686A) discloses a resin sheet obtained by coating at least one side of the sheet with a coating composition at a specific quantitative relationship, wherein the coating composition comprises an fatty acid amide, a polyoxyethylene-polyoxypropylene block copolymer, a silicone oil, and a water-soluble blue coloring agent, and the fatty acid amide and the polyoxyethylene-polyoxypropylene block copolymer constitute an antistatic agent.
These styrenic resin sheets provide anti-fogging property along with antistatic property. However, these styrenic resin sheets are also lack in enough anti-fogging property (particularly low-temperature anti-fogging property). Moreover, in order to increase mold-releasing property of the sheets, a relatively large amount of silicone oil is required.
Japanese Patent Application Laid-Open No. 46232/2002 (JP-2002-46232A) discloses a polystyrenic sheet in which one surface is coated with at least one anti-fogging agent selected from the group consisting of a sucrose fatty acid ester and a polyglycerin fatty acid ester at a coating amount of 5 to 30 mg/m2, and the other surface is coated with a mold-releasing agent comprising an ether-series polymer (a polyoxyethylene-polyoxypropylene block copolymer) and a non-block copolymeric ethylene oxide-series nonionic surfactant at a coating amount of 3 to 30 mg/m2. In this document, there is described that, in the case where a silicone oil is used, electrostatic charges are opt to generate by friction between a metal mold and the sheet. Japanese Patent Application Laid-Open No. 47366/2002 (JP-2002-47366A) discloses a polystyrenic sheet in which one surface is coated with at least one anti-fogging agent selected from the group consisting of a sucrose fatty acid ester and a polyglycerin fatty acid ester at a coating amount of 5 to 30 mg/m2, and the other surface is coated with a mold-releasing agent comprising an ether-series polymer (a polyoxyethylene-polyoxypropylene block copolymer, a polyethylene glycol) at a coating amount of 3 to 30 mg/m2 Japanese Patent Application Laid-Open No. 86639/2002 (JP-2002-86639A) discloses a polystyrenic anti-fogging resin sheet in which one surface is coated with an anti-fogging agent comprising at least one fatty acid ester selected from the group consisting of a sucrose fatty acid ester and a polyglycerin fatty acid ester, an ether-series polymer (a polyoxyethylene-polyoxypropylene block copolymer, a polyethylene glycol), and a polyvinyl alcohol, and the other surface is coated with a mold-releasing agent comprising an ether-series polymer (a polyoxyethylene-polyoxypropylene block copolymer, a polyethylene glycol). These documents describe that use of a silicone oil deteriorates anti-fogging property and induces generation of electrostatic charges by friction.
However, since the ether-series polymer is used as the mold-releasing agent, these anti-fogging sheets are significantly deteriorated in antiblocking property or mold-releasing property compared with a sheet using a silicone oil. Further, in the case where the resin sheet is rolled up in a roll form or contacted with a heated member such as a hot plate in the forming process, anti-fogging property and transparency are adversely affected, and the quality of a tray formed with the sheet becomes low. Incidentally, Japanese Patent Application Laid-Open No. 86639/2002 (JP-2002-86639A) describes, as a comparative example, a styrenic resin sheet in which one side is coated with an anti-fogging agent comprising a sucrose fatty acid ester, a polyoxyethylene-polyoxypropylene block copolymer, a polyvinyl alcohol, and a silicone oil, and the other side is coated with a mold-releasing agent comprising a polyoxyethylene-polyoxypropylene block copolymer and a polyoxyethylene sorbitan laurate.